Incorrect phpdoc typing

[gronostajo]

The method save takes an $image_type argument, which according to its phpdoc accepts a string. However the code expects it to be one of IMAGETYPE_* constants, which are integers. This causes static analyzers like PHPStan or Psalm to flag correct usage as erroneous. The fix is to simply update the phpdoc.

[adityapatadia]

Please create PR. Happy to merge.